In a typical visual inspection system for semiconductor wafers, all or a portion of a wafer is imaged onto a camera. The image captured by the camera is typically processed by software to determine the presence, absence, size and/or orientation of one or more particular features on the wafer. The particular features may vary, depending on the particular application of the inspection system. As used herein, the term “wafer” is to be construed to be inclusive of at least the following: an entire semiconductor wafer, a portion of a single die formed on a semiconductor wafer, a single die formed on a semiconductor wafer, multiple die formed on a semiconductor wafer, and portions or pieces of an entire semiconductor wafer (broken or sawn). The term “wafer” may also be construed as any substrate suitable for visual inspection. Further, the term “visual inspection” shall be construed to include any process or apparatus that involves the focusing of electromagnetic radiation of any wavelength at a given focal point or plane.
In general, it is desirable that the wafers be inspected quickly, so that a relatively large number of wafers may be inspected in a relatively short amount of time. There is continual effort to improve the efficiency of the inspection system algorithms and reduce the time required to inspect each wafer.
One step in a typical inspection system algorithm involves axially placing each wafer at the desired imaging plane, with respect to the imaging optics. Once the wafer is placed, the optics image the wafer or a portion of the wafer. In general, this axial location helps determine the quality of the image captured by the camera. It is a continual challenge to balance high throughput of the wafers through the inspection system (speed) versus accurate placement of each wafer at the most desirable imaging plane (accuracy).
Accordingly, there exists a need for increased efficiency in ensuring that each wafer is placed at a desired imaging plane, with respect to the imaging optics.